Add customer documentation for read/write/batch API operation performance enhancing separation

fineract-doc/src/docs/en/chapters/architecture/api-performance.adoc

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+= Introducing Performance Enhancing Read/Write/Batch API Operation Separation
+
+Fineract is introducing performance enhancing read/write/batch API operation
+separation.
+
+Now, Fineract has three different instance types:
+
+* Read instance
+* Write instance
+* Batch instance
+
+In cases where Fineract has to deal with high load, it can cause a performance
+problem for a single Fineract instance. To overcome this problem, Fineract
+instances can be started in different instance types for better scalability and
+performance in a multi-instance environment:
+
+Detailed documentation on instance types can be found in this document:
+https://github.com/apache/fineract/blob/develop/fineract-doc/src/docs/en/chapters/deployment/instance-type.adoc
+
+The linked document also explains deployment steps and specifications.
+
+= Background
+
+Instances, where we're planning to deploy Fineract to a highly-available setup
+with the highest-throughput because we can scale individual parts of it.
+
+Reading data happens much more often than actually writing data so the ability
+to do this type of deployment will greatly improve what we can achieve in terms
+of performance.
+
+Also, with the separation in place, we could utilize read-replica databases as
+well where write and batch instances connect to a master DB and the read
+instances connect to read-replicas therefore reducing the load on the database
+as well.
+
+All three instances come with different types of restrictions in terms of what
+they are capable of. For example a read instance is going to be only capable of
+serving read (GET) APIs. Write APIs will be able to serve both read and write
+APIs. Batch instance is only able to serve batch job related APIs and run the
+batch jobs themselves.
+
+= Context
+
+== Instance types
+
+**Read instance** only accepts `GET` request. Optionally, there are
+database connection details for this mode.
+
+**Write instance** accepts `GET/POST/PUT/DELETE` requests. It uses the
+original database connection details.
+
+**BATCH instance** accepts `POST` request related to run jobs. It will start the
+Quartz scheduler at startup time and run the batch jobs.
+
+== Modes
+
+* Full mode (default)
+* Read mode
+* Write mode
+* Batch mode
+
+= Setup
+
+== Environment Variables
+
+Using environment variables, the three instance types performance enhancing
+operation separation can be performed:
+
+* FINERACT_READ_MODE_ENABLED
+* FINERACT_WRITE_MODE_ENABLED
+* FINERACT_BATCH_MODE_ENABLED
+
+These three are the boolean variables to enable or disable the instance modes.
+
+== Database Settings
+
+Optional database connection details for reading mode are:
+
+.Database connection settings
+|===
+^| Key ^| Type ^| Value
+
+| schema_server | character varying (100) |  127.0.0.1
+| schema_name | character varying (100) | fineract_default
+| schema_server_port | character varying (10) | 5432
+| schema_username | character varying (100) | mifosadmin
+| schema_password | character varying (100) | password
+| readonly_schema_server | character varying(100) | [null]
+| readonly_schema_name | character varying(100) | [null]
+| readonly_schema_server_port | character varying(10) | [null]
+| readonly_schema_usernname | character varying (100) | [null]
+| readonly_schema_password | character varying (100) | [null]
+| readonly_schema_connection_parameters | text | [null]
+|===
+
+= Use Case Scenarios
+
+== APIs
+
+* Read APIs are allowed only for read and write instances
+* Write APIs are allowed only for write instances
+* Batch job APIs are allowed only for batch instances
+
+== Batch jobs
+
+* Batch job scheduling is allowed only for batch instances
+
+== Read-only instance type restrictions
+
+* Events are disabled for read-only instances
+* Read-only tenant connection support
+
+== Batch-only instance type restrictions
+
+* Receiving events is disabled for batch-only instances